A surface of a magnetic disk included in a hard disk drive (HDD) is provided with an area to which servo data (position data) is written, and an area to which user data is written. The hard disk drive writes/reads user data by accessing a specified area on the basis of servo data read out by a head. Here, the positioning control of positioning the head at a specified area is achieved by the feedback control, which includes the steps of: generating a position error signal, which indicates the amount of a position error of the head, from the servo data read out by the head; and on the basis of the position error signal, controlling a voice coil motor used for moving a position of the head.
However, in the case of hard disk drives, disturbances act on the feedback control that is performed to achieve positioning due to various factors (for example, the center of a magnetic disk deviates from the rotation center of a spindle motor), and consequently disturbance components caused by the disturbances are added to a position error signal. This makes it difficult to position a head. Because the disturbances are caused by various factors, it is not possible to identify frequencies of the disturbance components included in the position error signal beforehand. Therefore, it is difficult to effectively eliminate such disturbance components.
For this reason, Japanese Patent Publication No. 2003-109335 (“patent document 1”) discloses a hard disk drive that includes a digital filter capable of changing a setting frequency whose gain becomes effective, and that eliminates a disturbance component by causing a setting frequency of the digital filter to follow a frequency of the disturbance component included in the position error signal. Here, if a phase of the position error signal coincides with a phase of an output signal of the digital filter, the position error signal is lower in frequency than that of the output signal of the digital filter. On the other hand, if the phases are shifted from each other by 180 degrees, the position error signal is higher in frequency than that of the output signal of the digital filter. By comparing both of them on the basis of the above-described facts, a judgment is made as to whether the setting frequency of the digital filter should be increased or decreased, and thereby the setting frequency of the digital filter is controlled so that the setting frequency follows the frequency of the disturbance component.
However, in the case of such a digital filter, because a setting frequency is stored in a register having the specified bit length, it is not possible to set a frequency which can be expressed with this bit length and which is out of a range. In addition, because the setting frequency is expressed as a digital value, the value becomes discrete. Therefore, the sufficient resolution may not be achieved. In this case, properties of the digital filter may not effectively act on the disturbance components included in the position error signal.